Autodeposition, also known as Autophoresis.RTM. or chemiphoresis, is a process for depositing an organic coating on a metal surface. The process involves the controlled release of multivalent metal ions from the metal surface which destabilizes dispersed polymer particles in the coating composition such that a coating builds up on the metal surface. Such systems are well know in the coating art and are described in for example, U.S. Pat. Nos. 3,776,848; 3,791,431; 4,108,817; 3,839,097; 3,829,371; 4,104,424 and 4,030,945.
Autodeposition, though a relatively new procedure for coating metal surfaces, has achieved considerable importance and wide use in the coating industry due to its many advantages over conventional systems, such as electrolytic deposition. By using autodeposition, practitioners of the art can now deposit an inorganic and an organic film simultaneously with fewer operating steps in less time while utilizing less floor space than conventional pretreatment/electrocoating systems. Autodeposition also reduces the air and water pollution associated with conventional coating systems because organic solvent usage is minimized. The use of autodeposition also significantly reduces the energy usage required by certain conventional electrocoating systems. A further advantage is the sharply decreased safety risk attained by avoiding the use of electric current in the coating bath.
A major disadvantage of autodeposition coatings is the very great difficulty in obtaining an autodeposition coating which will crosslink upon curing. It is believed that this difficulty is due to the low pH which is required for autodeposition. Conventional crosslinkers, when introduced to the highly acidic autodeposition coating compositions tend to cause destabilization of the autodeposition composition. Since crosslinking generally has a beneficial effect on the characteristics of the cured coating, an autodeposition coating composition which can remain stable as a coating bath and which will crosslink upon curing would be of great advantage.